Post harvest diseases fruits and vegetables - Xavier University ...

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Chemical Control 183 may form a protective barrier on the surface of the fruit and reduce the supply of oxygen for enzymatic oxidation of phenohcs (Zhang and Quantick, 1997). Apphcation of a chitosan coating also delayed, to some extent, the increase in decay of stored litchi. Chitosan application was also found to stimulate the formation of structural defense barriers in bell peppers and tomatoes. These included the induction of callose synthesis, thickening of host cell walls, formation of papillae and plugging of some intercellular spaces with fibrillar material, probably impregnated with antifungal phenolic-like compounds (Wilson et al., 1994). However, chitosan also exhibits a direct fungicidal activity and can affect postharvest pathogens by suppressing their growth: it inhibited spore germination, germ-tube elongation and radial growth of B, cinerea and K stolonifer, inducing cellular alterations and damage in culture (El Ghaouth et al., 1992). Similarly, exposure of Sclerotinia sclerotiorum to chitosan resulted in hyphal deformation and restricted mycelium growth (Cheah et al., 1997). Trials with chitosan-treated bell peppers indicated the damage caused to B. cinerea hyphae, exhibited as changes in the cell wall and by cytoplasm decomposition (El Ghaouth et al., 1994). Examination of sections from chitosan-treated tissue of bell pepper fruits by transmission electron microscope revealed that chitosan prevented the disintegration of host cell walls by the pectolytic enzymes of B. cinerea (El Ghaouth et al., 1997). This was indicated by the preservation of pectic binding sites and the regular intense cellulose labeling over host cell walls pressed against fungal cells. In addition to reducing the production of polygalacturonase by the pathogen, chitosan also caused severe cytological damage to invading hyphae. These phenomena may explain, at least in part, the limited ability of Botrytis to colonize the fruit tissues in the presence of chitosan (El Ghaouth et al., 1997). In parallel, chitosan may also induce the formation of antifungal hydrolases, such as chitinase and P-l,3-gluconase, which may lead to the reduction in the chitin content of fungal cell walls and the stimulation of various structural defense barriers in fruits such as bell peppers and tomatoes. Because of these characteristics, chitosan has been hypothesized to be an elicitor of resistance in harvested crops (see the chapter on Novel Approaches for Enhancing Host Resistance - Induced Resistance). Gel derived from Aloe vera plants has been found to have antifungal activity against four common postharvest pathogens: Penicillium digitatum, P. expansum, B. cinerea and A. alternata. The natural gel http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE 184 Postharvest Diseases of Fruits and Vegetables suppressed both germination and mycelial growth, with P, digitatum and A. alternata being the most sensitive species. The antifungal potential of the gel in decay suppression was exhibited on P. digitatum-inoculated grapefruit, and was expressed in a delay in lesion development as well as a significant reduction in infection incidence at shelf life conditions (Saks and Barkai-Golan, 1995). Latex, present in some fruits, is another natural fungicide or a source of fungicides, which is regarded as both safe and effective against various diseases of banana, papaya and other fruits (Adikaram et al., 1996). Papaya latex is a complex mixture of sugars with several enzymes, notably proteases, glucosidases, chitinases and lipases. The water-soluble fraction of papaya latex can completely digest the conidia of many fungi, including important postharvest pathogens (Indrakeerthi and Adikaram, 1996). A small cystein-rich protein, he vein, was isolated from the latex of the rubber tree (Hevea brasiliensis). It showed a strong antifungal activity in vitro against several fungi such as B. cinerea and species of Fusarium and Trichoderma (van Parijs et al., 1991). Hevein, which is a chitin-binding protein, might interfere with fungal growth by binding or cross-linking newly synthesized chitin chains. Alternatively, the delicate balance between chitin synthesis and selective hydrolysis of preformed chitin chains in hyphal walls might be interrupted (Cabib, 1987; van Parijs et al., 1991). Antibiotic compounds secreted by antagonistic bacteria are also among the natural compounds which may suppress postharvest pathogen development. Such compounds are the iturins, antimicrobial substances produced by Bacillus subtilis strains or isolates. They have a wide antifungal spectrum and were found effectively to control M. fructicola development in cold-stored stone fruits (Pusey, 1991). Pyrrolnitrin is a natural antibiotic compound produced by Pseudomonas cepacia. This compound was found to delay JB. cinerea development in strawberries, although no reduction of the total incidence of decay was recorded (Takeda et al., 1990). F. LECTINS Lectins are a class of sugar-binding proteins that are widely distributed in nature and their occurrence in plants has been known http://arab2000.forumpro.fr
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